1. Field of the Invention
This invention relates in general to airport surveillance systems for determining the position and identification of aircraft on the field which utilizes secondary radar including an interrogation station with a narrow transmitting beam and includes one or more receiving stations mounted at different locations on the airport which receive transponded signals from the aircraft.
2. Description of the Prior Art
It is intended that in the future airports will be equipped with a new landing system which allows air traffic to land and take off even under very poor visibility conditions. The aircraft controllers must also ensure the proper and safe operation of aircraft traffic on the landing field before it has landed and after it has landed under conditions of poor visibility where often times they may not be able to visually see and identify the aircraft. This makes it necessary that a system be installed which allows surveillance of the movement and operation of the aircraft on the flying field under poor visibility conditions and requires that the positions and the identity of the aircraft are continuously presented on a suitable display means.
Several large airports are equipped with high resolution primary radar such as X-band radar systems for the surveillance of the airfield. However, such high resolution radar has the disadvantage that when the wave length becomes shorter, the detection and discovery probability particularly in the event of heavy rainfall is not sufficient and also such systems do not provide for the identification of the aircraft.
It is also possible for airfield surveillance to install on the runways sensors as, for example, inductive loops, leader cables, radar sensors, infrared or pressure sensors. However, sensors of this type monitor only the runways and not the remaining portion of the fields such as the taxiways and entail very high expenses for the systems.
Airfield surveillance can also be accomplished with secondary radar. When using secondary radar, the position as well as the identification on the entire airport field is possible. However, difficulties arise because with existing secondary radar surveillance systems the reply messages have a comparatively long duration so that the replies of the transponders mounted on the aircraft overlap within the airfield.
An airport surveillance system from Great Britain is discussed in "AGARD Conference Proceedings," No. 188, Article 21 and describes secondary radar system which in order to resolve a reply from the airplane transponder utilizes two pulses (P.sub.1, P.sub.3) with a defined time spacing. On the airfield, two transmitting stations are installed and one emits the pulse P.sub.1 with a directional antenna and the other transmitting station emits the pulse P.sub.3 with an omni-directional antenna pattern. At specified locations on the airfield, the time condition for the acceptance of the interrogation in the transponder is satisfied. If an airplane is located at that instant at such location, then its transponder will transmit a reply. By using this technique, the airfield can be scanned such that always only one transponder replies. A receiver located at the edge of the airfield decodes the reply and the position of the airplane can be determined from the angular position of the directional radiated beam and the adjusted time intervals between the two pulses P.sub.1 and P.sub.3. In the English system, the precision of the location of the aircraft can be improved by utilizing hyperbolic techniques. Since the area in which the transponder of the aircraft is relatively large, overlapping can occur from replies from more than one aircraft.
A U.S. airport surveillance system described in "AGARD Conference Proceedings," No. 188, Article 22 utilizes secondary radar for the selection of the individual airplanes and utilizes Ingterrogation Path Side Lobe Suppression (ISLS). If the amplitude of the reference level pulse P.sub.2 is greater than the amplitude of the first interrogation pulse P.sub.1 then no reply will be transmitted by the transponder on the respective aircrafts. Two interrogation stations scan the airfield using monopulse antennas. Only at the intersection point of the two beams is the amplitude of the pulse P.sub.2 smaller than the amplitude of the pulse P.sub.1 so only from this location will a transponded reply be emitted. Position determination is accomplished with the use of receivers wherein the transit time or time delay differences of the received signals are detected and used to calculate position. Due to reflections which cause the level of the pulse P.sub.2 to be greater than that of pulse P.sub.1 such as from other aircraft or buildings, areas exist in which it is not possible to locate the aircraft.
Other systems which function with the selection of individual airplanes using ISLS techniques are described in the journal "Aviation Week & Space Technology," of June 28, 1976, pages 67 through 73. Also, the journal "Electronics," of May 13, 1976, page 34 discusses a system. See also German OS 2,538,382. These airport surveillance systems have areas in which location of aircraft is not possible.